Mormon’s letters

I never before considered this but the letters from Mormon to Moroni must have been written on something less durable than plates. Whether it was paper or leather or even wood or clay tablets I have to wonder how Moroni was able to copy those letters to plates decades later. My best guess would be that he saved those letters with other records wherever those records were stored. If that was the case then when he found that he could safely access those records he proceeded to fill the remainder of the plates with whatever was most precious among those records (of all types of material) which had not already been covered in the plated record.